1. Field of the Invention
The present invention relates to triangular wave generation circuits including a rectangular wave generation circuit for generating an intermittent rectangular wave signal in which a rectangular wave interval and a direct current interval of a predetermined level are repeated, and an integration circuit for generating an intermittent triangular wave signal in which a triangular wave interval and the direct current interval are repeated based on a reference signal and the intermittent rectangular wave signal generated by the rectangular wave generation circuit, in particular, to a triangular wave generation circuit, mounted on a vehicle for the purpose of measuring distance between cars, for generating a frequency modulated signal in an FM-CW radar capable of measuring the relative speed and the distance with respect to a target object.
2. Description of the Related Art
Conventionally, the intermittent triangular wave signal in which the triangular wave interval and the direct current interval are repeated is generated as the frequency modulated signal by the integration circuit using an operational amplifier in which the reference signal is input to the non-inverted input terminal and the intermittent rectangular wave signal in which the rectangular wave interval and the direct current interval of a predetermined level are repeated is input to the inverted input terminal, as shown in FIG. 13.
The triangular wave generation circuit as disclosed in Japanese Laid-Open Patent Publication No. 62-152214 has been proposed for such triangular wave generation circuit, which triangular wave generation circuit removes the offset generated in the generated triangular wave by interposing a capacitor in the output circuit.
However, the reference signal is generated by resistance dividing the power supply voltage in the conventional integration circuit described above, and thus the influence caused by variation of the power supply voltage is significant, and in addition to the generation of the offset in the triangular wave generated by the integration circuit, the output signal of the integration circuit drifts either upward or downward since the reference signal level and the signal level of the direct current interval of the intermittent rectangular wave signal are different, whereby waveform distortion occurs in the intermittent triangular signal to be output, as shown in FIG. 14.
If the intermittent triangular wave signal output from such triangular wave generation circuit is used as the frequency modulated signal in the FM-CW radar and the like, the error of the detecting target becomes large, and may greatly influence various controls that are based on the relevant output.
Furthermore, since the above problem also arises due to external factors such as temperature characteristic of the integration circuit in addition to flaws in circuit design, the frequency and the size of the waveform distortion become irregular, and a stable triangular wave generation cannot be performed by simply making modifications on the circuit design such as adding a capacitor etc.